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[Crankyman]

I'm not really looking for a fix here but I'll listen to any suggestions anyway.  It's really just for my own understanding.

I'm on vdsl in the wilds of North Norfolk, 36 down is as fast as it gets, I was using a Draytek Vigor 130 in bridge mode to a Synology MR2200AC router but was having problems with IPv6.  Everything would connect initially and be Ok, but after 5 mins or so IPv6 would drop to limited connection and cause all sorts of problems with various phone apps and my Google devices amongs others.

Going into the settings on the Synology if I unticked the "set as default gateway" box the connection would return, but only for about 5mins or so again, back into the Synology and the tick had returned into the "set as default gateway" box, untick the box, connection comes back.... and, well you get the picture.  IPv4 was Ok throughout all this so I still had interweb access.

I raised a support case with Synology, who were pretty good on the whole, apart from some idiot who suggested I turn off IPv6! which is what I was trying to get working in the first place.  Several times they told me my modem wasn't compatible with IPv6 or it was causing the problem and I quoted parts of the Draytek spec to them and explained when it worked it was Ok and posted screen dumps to prove it, their router was changing the default gateway all by itself.

I had an OR ECI modem in my collection of "stuff" in the shed so to satisfy them I installed it.  Lo and behold it worked, the default gateway remained stable and IPv6 stayed connected.
Replied back to Synology with a good helping of humble pie!!

Now I thought pass through or bridge mode meant everything got demodulated and sent to the router, but something must be changing in the Draytek to cause the problem.
I've since "upgraded" to an Huawei HG612 and that works fine too, every time I reconnect the Draytek the IPv6 problem returns, this has happened through the last 3 firmware upgrades too, currently on 3.8.5.1 BT

Either my understanding of bridge mode is wrong or the Draytek config is wrong, not that there's much to config on the Draytek.
Does anyone understand what's happening, I certainly don't 

[tubaman]

This may be of interest - https://forums.whirlpool.net.au/archive/92kx2y43
It is a few years old but one user appears to have got it working and another is being told by Draytek that the Vigor 130 doesn't support ipv6 bridging at all. 

[Alex Atkin UK]

I really do hate it when bridge mode is not true bridge mode as yes it should just blindly forward everything.

[Crankyman]

Thanks Tubaman, that's an interesting link.

Thanks Alex, for confirming what I thought should happen, even though it doesn't

[skyeci]

I ran a vigor 130 in bridge on fttc with zen  and ipv6 worked fine with my opnsense box. Was like this for some considerable time pre shifting to fttp.

[Crankyman]

I don't really understand?  The problem appears to be to do with the gateway address, when my router initally establishes a connection the gateway is received from my ISP (Now (Sky)) but subsequently changes and that's what causes the problem.

Maybe it's to do with the combination of Draytek modem and Synology router? or perhaps RIP on one device or the other is receiving some different information.

I haven't been able to access the UI for the modem without disconnecting the router to investigate further, not that I've tried hard to do that. Like most things, don't mess with it, it's working!
Or should that be if it ain't broke keep messing with it until it is 

[skyeci]

I had the same issue with opnsense on a relatives line which was using a different modem but opnsense kept dropping ipv6, this was with sky also... I never got to the bottom of it so even now ipv6 is off on that setup.

[Crankyman]

My IPv6 worked with my ECI modem and is currently working with my Huawei 612, it's just the Draytek 130 that gives problems, but the Draytek is the one that performs best overall 

Sky's provided router works Ok too but the WiFi is very poor and it is sadly lacking in features as is the way with most ISP provided routers.

I don't like to keep chopping and changing modems about in case something throttles my already slow "High speed broadband"